The present invention relates to a mixing capsule for dental compositions having a mixing chamber in which a piston is slidably guided whereby within the piston a plunger is slidably arranged and wherein the mixing chamber is provided with an outlet opening via which the dental composition can be forced out of the mixing chamber.
From German Offenlegungsschrift 20 60 262 such a mixing capsule for dental compositions is known. Such a mixing capsule is provided with a hollow piston in which a plunger is guided whereby the hollow piston, in general, contains a liquid substance which is separated from a powdery substance by a foil that is capable of being torn whereby the powdery substance is contained in the mixing chamber. By depressing the plunger the foil is torn and the generally liquid substance is pressed out of the hollow piston and also introduced also into the mixing chamber so that the substances can be mixed within an automatic oscillating mixing device, for example, a so-called amalgam mixer.
It is important to realize that the piston upon depressing the plunger should not also be forced into the mixing chamber, not even to the smallest extent. Otherwise there is the danger that the mixing chamber volume is decreased such that a complete mixing of the components, for example, of a fastening cement, is possible only to a limited extent and that the cement will suffer an unacceptable quality loss.
For solving this problem it is suggested in German Offenlegungsschrift 20 60 626 to provide the piston with lateral projections whereby the depression of the plunger is carried out such that the projections provide a kind of counter grip which can be held with two fingers while the plunger is provided with a thumb rest.
This solution in principle is favorable since this arrangement corresponds to the natural hand position used for depressing a syringe. Since the mixing chamber itself also has corresponding projections for emptying there is the risk that when an operator is careless or is in training for using this apparatus, that accidentally the projections of the mixing chamber are used instead of the projections of the hollow piston as counter grips, which mistake may even remain undetected.
In addition, it is also known, for example, from German Offenlegungsschrift 20 24 331 and German Offenlegungsschrift 19 39 316, to support a hollow piston at the mixing chamber with a support or spring ring. The spring ring is removed with a conventional spring ring remover before removing the finish-mixed tooth cement or amalgam so that the hollow piston is freely displaceable and can be depressed.
In this solution it is disadvantageous that the piston together with the liquid contained therein is not secured in an optimal manner. The plunger is provided with sealing lips relative to the hollow piston so that no liquid can emerge. However, when the plunger is already depressed, depending on how the device is handled, a relative movement between piston and mixing chamber may occur, especially since the entire capsule is greatly accelerated within the amalgam mixer, and even when this relative movement is relatively small, it may happen that liquid leaks into the gap between the mixing chamber and the piston which would result in undesirable contamination.
A plurality of other mixing capsules has also been suggested whereby the capsule according to German Offenlegungsschrift 37 18 326 shall be mentioned as an example. In this mixing capsule a piston is always completely received within the mixing chamber. From the exterior it is not easily detectable in which state the mixing capsule is at the moment, whether, for example, the foil container containing the liquid is already broken or whether the piston is at this point simply depressed to some extent. The capsule is only secure when it is positioned in an actuating device. However, when in practice a plurality of capsules needs to be prepared, it is required to purchase a plurality of actuating devices. Furthermore, there is a risk that, when the capsule is inserted in a conventional amalgam mixer after depressing the plunger, the piston is moved inwardly to a certain extent so that the gap between piston and plunger reaches the area of the engagement opening which would also lead to contamination.
It is therefore an object of the present invention to provide a mixing capsule of the aforementioned kind which in practice can be operated securely and prevents contamination without the need for replacement of conventional automatic oscillating devices with new constructions.